The present invention is directed, in general, to electronic devices and, more specifically, to a battery charger for a battery-powered electronic device and a method of operating the same to charge a battery more precisely under varying input voltage conditions.
The number of battery-powered electronic devices is rapidly increasing due to user demands for higher mobility and broader general utility. This has resulted in a higher demand for improved performance from the batteries to allow the electronic devices to be operationally available for a greater percentage of the time. Conventional battery monitoring and conditioning systems are focused on simplicity of operation regardless of any degenerative impact that the systems may have on the batteries themselves. For example, a conventional battery monitoring and conditioning system may provide a fixed charging current during the charging period. This fixed charging current is typically maximized to assure that the batteries are charged quickly. Alternatively, a battery may be charged only by a trickle current, which may not damage the battery but will require an unacceptably long period of time to charge a fully discharged battery.
A few parameters that are detrimental to batteries include a large or uncontrolled charge current, a constant value of charging voltage that is too high, a high battery temperature that causes the battery to vent gas, shorted battery cells that force other cells to overcharge, and conditions that cause the battery to be discharged beyond a certain minimum threshold. A maximum charging current is often limited only by the internal resistance of the batteries themselves. A large battery charging current produces a high likelihood that the batteries will overheat. Overheating of the batteries causes the internal resistance of the batteries to decrease further, thereby allowing the charging current to further increase. This process, if unchecked, often produces a thermal runaway condition that either partially or severely damages the batteries.
Even if the batteries do not enter a thermal runaway condition, a xe2x80x9cthermal eventxe2x80x9d may exist wherein the batteries becomes overheated and vents gas to the environment, causing the batteries to dry out. Over time, battery cells that have been damaged by a charging process may become shorted. A shorted battery cell can no longer properly store energy, which forces other battery cells in the battery to overcharge. This condition usually requires that the battery be removed and discarded. Although simple in operation, the conventional battery conditioning system treats all batteries in the same way and usually monitors the voltage condition of the battery only to determine the presence of a shorted cell.
Accordingly, what is needed in the art is an improved system and method of charging a battery that allows a battery to be charged at an appropriate level while avoiding detrimental conditions such as the thermal event described above.
To address the above-discussed deficiencies of the prior art, the present invention provides, for use in a battery-powered electronic device, a battery charging circuit, a method of charging a battery and a battery-powered electronic device employing the circuit or the method. In one embodiment, the battery charging circuit includes a charging switch, coupled between a base interface of the battery-powered electronic device and a battery thereof to be charged. When closed, the charging switch provides a conductive path for charge current from the base interface to the battery. The charge current is based on an input voltage of the battery charging circuit and a voltage of the battery. The battery charging circuit further includes a controller that modulates the charging switch at a duty cycle that at least in part determines a rate at which the charge current is delivered to the battery thereby to compensate at least in part for variations in the input voltage.
The present invention introduces, in one aspect, the concept of providing a battery charging circuit that employs a charging switch associated with a battery-powered electronic device to charge the battery in a more accurate manner. The battery charging circuit is sensitive to both its input voltage and the actual voltage of the battery. This allows the battery charging circuit to apply an appropriate charge current that will charge the battery in an effective manner while limiting the possibility of battery damage due to overcharging. This strategy provides a minimal charging time while protecting the battery. Additionally, the present invention can advantageously employ portions of a controller that are already in place to control the electronic device.
In one embodiment of the present invention, the battery charging circuit further includes a trickle charge resistor, coupled in parallel with the charging switch, that provides a trickle charging conductive path for a trickle charging current. The trickle charging current represents a minimum value of charge current for the battery. The value of the trickle charge resistor may preferably be selected to establish an appropriate trickle charging current to minimize the needed modulation of the charging switch.
In a related embodiment of the present invention, the controller includes a processor that modulates the charging switch based on the trickle charging current. The trickle charging current indicates a condition of charge on the battery thereby allowing the modulation of the charging switch to be adjusted as appropriate to optimize the charging of the battery.
In one embodiment of the present invention, the controller includes a processor that modulates the charging switch based on the input voltage. This allows the duty cycle of the charging switch to be increased or decreased to compensate for variations in the input voltage, thereby providing an appropriate charging rate.
In one embodiment of the present invention, the controller includes a processor that modulates the charging switch based on the voltage of the battery. Monitoring the voltage of the battery allows the charge of the battery to be determined. The charging current may then be adjusted to optimize the charging response of a particular battery.
In one embodiment of the present invention, the controller contains a preprogrammed set of discrete duty cycles for the charging switch that establishes a corresponding set of discrete charging rates. In an embodiment to be illustrated and described, the electronic device controller contains ten different discrete duty cycles, evenly spaced and based on a period of one second. Alternatively, any number of different discrete duty cycles may be employed, and the period may be adjusted to accommodate a specific application.
In one embodiment of the present invention, the battery-powered electronic device is a cordless telephone. Those skilled in the pertinent art will understand, however, that the principles of the present invention may be applied to any battery-powered electronic device.
The foregoing has outlined, rather broadly, preferred and alternative features of the present invention so that those skilled in the art may better understand the detailed description of the invention that follows. Additional features of the invention will be described hereinafter that form the subject of the claims of the invention. Those skilled in the art should appreciate that they can readily use the disclosed conception and specific embodiment as a basis for designing or modifying other structures for carrying out the same purposes of the present invention. Those skilled in the art should also realize that such equivalent constructions do not depart from the spirit and scope of the invention in its broadest form.